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Never used that type but darkening isn't burning I wouldn't worry about it if it's just BT and nothing else. You can google product+MSDS and look it up yourself. Think of a bacteria organism. When it dries it basically disappears for the most part over time. I have never observed this darkening effect but I imagine that it would go away. It's nowhere near the problem of crop loss / diminishment in quality from caterpillars and botrytis. If your intuitions telling you to cut the dose then do it. You could say plants speak to us in this way.